Pulsed ion beam source

Abstract

An improved magnetically-confined anode plasma pulsed ion beam source. Beam rotation effects and power efficiency are improved by a magnetic design which places the separatrix between the fast field flux structure and the slow field structure near the anode of the ion beam source, by a gas port design which localizes the gas delivery into the gap between the fast coil and the anode, by a pre-ionizer ringing circuit connected to the fast coil, and by a bias field means which optimally adjusts the plasma formation position in the ion beam source.

I claim:  
     
       1. A pulsed ion beam source comprising: 
       means for delivering a gas puff to a first gap between fast magnetic field means for forming a fast magnetic field and anode electrode means, the fast magnetic field fully ionizing the gas puff to produce a plasma and moving the resulting plasma into an accelerating gap between the anode electrode means and cathode electrode means;  
       means for forming a magnetically insulated accelerating gap for passing the plasma comprising means for forming a slow magnetic field, said slow magnetic field means being located proximate to the cathode electrode means;  
       wherein the fast and slow magnetic fields intersect to define a separatrix region where the plasma is on a flux surface that eliminates beam rotation and  
       means for delivering a positive electric charge pulse from a power source to the anode electrode means for the purpose of accelerating ions in the plasma from the separatrix region towards a target.  
     
     
       2. The ion beam source of claim  1  wherein the anode electrode means and the cathode electrode means are ring-shaped structures disposed about a central axis of the ion beam source with each such structure having an annulus through which the plasma passes. 
     
     
       3. The ion beam source of claim  2  wherein said means for delivering a gas puff allows supersonic flow rates within it and causes localized delivery of the gas puff into the first gap but without entry of the gas puff in an nonionized state into the annulus of the anode electrode means. 
     
     
       4. The ion beam source of claim  2  wherein the structure of the anode electrode means is optimized to locate the separatrix within the anode electrode annulus. 
     
     
       5. The ion beam source of claim  1  wherein the fast magnetic field means further includes means for positioning the plasma within the first gap. 
     
     
       6. The ion beam source of claim  1  further comprising means for preionizing the gas puff. 
     
     
       7. The ion beam source of claim  4 wherein thefurther comprising a means for preionizing the gas puff comprisescomprising said fast magnetic field means and a capacitor which formsform a tingingringing circuit with the fast magnetic field means withhaving a period of less than one microsecond. 
     
     
       8. The ion beam source of claim  1  wherein the fast magnetic field means and the slow magnetic field means comprise, respectively, fast and slow magnetic coils. 
     
     
       9. The ion beam source of claim  8  further comprising bias field means including a bias field capacitor connected to the fast magnetic coil to produce a greater than one microsecond risetime current in the fast magnetic coil prior to full  pre-ionization of the gas puffy , puff said bias field optimizing  positioning the position of the  gas for pre-ionization of the gas puff  into the  a partially ionized plasma. 
     
     
       10. The ion beam source of claim  8 wherein none ofwhere essentially no magnetic flux from the slow magnetic field means are located on the side of the first gap proximate to the fast magnetic coil field means. 
     
     
       11. A magnetically confined anode plasma source for generating an ion beam having little or no rotation, said plasma source comprising: 
       
         a cathode assembly having a cathode electrode and an associated slow magnetic coil field for generating a magnetic flux having a first profile;  
       
       
         an anode assembly having an anode electrode and an associated fast magnetic field coil for generating a magnetic flux having a second profile, said first and second magnetic flux profiles having a separatrix when said slow and fast magnetic field coils are energized;  
       
       
         wherein said cathode electrode and anode electrode form therebetween an ion acceleration gap and wherein the separatrix between said first magnetic flux profile and said second magnetic flux profile is disposed in said acceleration gap adjacent to said anode electrode.  
       
     
     
       12. The plasma source according to claim  11 , wherein said slow magnetic field coil has a rise time of about  1  ms.  
     
     
       13. The plasma source according to claim  11 , wherein the magnetic flux of said slow magnetic field coil provides magnetic insulation for the ion acceleration gap everywhere.  
     
     
       14. The plasma source according to claim  11 , wherein said first magnetic coil has a rise time of less than about  1  μs.  
     
     
       15. A magnetically confined anode plasma source for generating an ion beam comprising: 
       
         an anode assembly including an anode electrode having an associated fast driving coil;  
       
       
         a cathode assembly including a cathode electrode and an associated slow driving coil, said anode electrode and cathode electrode forming therebetween an ion acceleration gap;  
       
       
         a gas valve assembly associated with said anode assembly, said gas valve assembly including a supersonic gas delivery nozzle for delivering a localized volume of gas to a region adjacent to said fast driving coil, said gas volume, as delivered to said region, being essentially free of flow components transverse to said acceleration gap.  
       
     
     
       16. The plasma source according to claim  15 , wherein said supersonic nozzle directs said gas volume only in the axial direction of said nozzle and deposits essentially all of said gas volume in the region adjacent to said fast coil.  
     
     
       17. The plasma source according to claim  16 , wherein said anode electrode has an edge portion and said supersonic nozzle includes an enlarged opening which opens into the region adjacent said fast coil and proximate to the edge portion of said anode.  
     
     
       18. The plasma source according to claim  16 , wherein said gas valve assembly includes gas plenum and a flapper valve for controlling introduction of the gas from said supply plenum to said supersonic nozzle.  
     
     
       19. The plasma source according to claim  18 , wherein said flapper valve is magnetically actuated.  
     
     
       20. A magnetically confined anode plasma source for generating an ion beam comprising: 
       
         an anode assembly including an anode electrode and an associated fast driving coil;  
       
       
         a cathode assembly including a cathode electrode and an associated slow driving coil, said anode electrode and cathode electrode defining therebetween an ion acceleration gap;  
       
       
         a gas valve assembly for introducing a volume of gas into a region adjacent said fast driving coil upstream of said acceleration gap;  
       
       
         a fast coil energizing circuit including a voltage source, switchably connected to said fast driving coil, for ionizing the gas to form a plasma and for adjusting the magnetic field between the fast driving coil and the anode electrode to thereby position the volume of plasma with respect to the accelerating gap.  
       
     
     
       21. The plasma source according to claim  20 , wherein said fast coil energization circuit further comprises a second voltage source connected in parallel with said fast coil for forming, when together with said fast driving coil, a ringing circuit for pre- ionizing the gas positioned between the fast driving coil and the anode electrode.    
     
     
       22. The plasma source according to claim  21 , wherein said ringing circuit has an oscillation period of less than about  1  μs.  
     
     
       23. The plasma source according to claim  21 , wherein said fast coil energizing circuit is switchably connected to said fast driving coil for directing the pre- ionized gas away from said fast driving coil, and a power source for energizing said slow driving coil whereby the resulting magnetic field provides a stagnation zone for said pre - ionized gas.    
     
     
       24. The plasma source according to claim  20 , wherein said voltage source comprises a current supply connected in parallel with said fast driving coil to adjust the magnetic field to improve plasma formation and positioning.  
     
     
       25. The plasma source according to claim  24 , wherein said current supply comprises a bias capacitor.  
     
     
       26. A pulsed magnetically confined anode plasma source for surface treatment comprising: 
       
         an anode assembly including an anode electrode and an associated fast driving magnetic coil;  
       
       
         a cathode assembly including a cathode electrode and an associated slow driving magnetic coil, said anode electrode and cathode electrode forming therebetween a diode having an ion acceleration gap, said slow driving magnetic coil forming a diode insulating magnetic field in said plasma acceleration gap;  
       
       
         a gas valve assembly for introducing a volume of gas into a region adjacent said fast driving coil;  
       
       
         an inductively produced electric field producing source for ionizing said gas into a plasma; and  
       
       
         a pulsed power source connected to said anode assembly for accelerating ions from said plasma to form an extractable ion beam, said anode electrode being configured to shape the magnetic flux from the fast driving and slow driving coils whereby said ion beam is extracted with little or no rotation.  
       
     
     
       27. The plasma source according to claim  26 , wherein said anode electrode is configured to provide a separatrix between the magnetic fluxes from said slow during coil and fast driving coil at the time of beam extraction.  
     
     
       28. The plasma source according to claim  27 , wherein said separatrix is positioned adjacent to said anode electrode in said plasma acceleration gap.  
     
     
       29. The plasma source according to claim  27 , further comprising an essentially magnetic field free region downstream from said cathode assembly in the direction of ion beam propagation, through which said extracted ion beam propagates.  
     
     
       30. The plasma source according to claim  29 , wherein said essentially magnetic field free region terminates at a surface of a workpiece to be treated and extends at least  20  cm in the direction of beam propagation.  
     
     
       31. The plasma source according to claim  26 , wherein said slow driving coil is disposed essentially exclusively in said cathode assembly.  
     
     
       32. The plasma source according to claim  26 , wherein said plasma source is maintained at a vacuum of about  10   −3    torr.    
     
     
       33. The plasma source according to claim  26 , wherein said extracted ions are drawn essentially exclusively from said plasma and said pulsed power source is operable to run continuously at a pulse rate of up to about  120  Hz.  
     
     
       34. The plasma source according to claim  26 , wherein said gas valve assembly includes a selectable source of gas whereby the composition of the ions in the extracted ion beam are controllable.  
     
     
       35. The plasma source according to claim  34 , wherein said gas source is selected from the group comprising hydrogen, helium, oxygen, nitrogen, fluorine, neon, chlorine, argon, lithium, beryllium, boron, carbon, sodium, magnesium, aluminum, silicon, phosphorous, sulfur and potassium.  
     
     
       36. A method of generating an ion beam using a magnetically confined anode plasma ion source comprising a vacuum chamber having an anode assembly including an anode electrode and a fast driving coil and a cathode assembly including a cathode electrode and a slow driving coil, the anode electrode and cathode electrode defining therebetween an acceleration gap, said method comprising the steps of: 
       ( a )  introducing a puff of gas into said vacuum chamber to produce a localized volume of gas adjacent said fast driving coil but insulated from said fast driving coil;    
       ( b )  pre - ionizing said gas using said fast driving coil;    
       ( c )  further ionizing said gas while moving said gas away from said fast driving coil to create a thin, magnetically confined plasma layer;    
       ( d )  applying a pulsed power signal to said anode electrode to form an ion beam from said plasma; and    
       ( e )  extracting said ion beam essentially without rotation thereof.    
     
     
       37. The method according to claim  36 , wherein the step of introducing a gas puff further comprises rapidly introducing said gas puff in a time interval depart of the gas used.  
     
     
       38. The method according to claim  36 , wherein the step of introducing a gas puff further comprises delivering said gas puff supersonically.  
     
     
       39. The method according to claim  38 , wherein the step of introducing a gas puff comprises expanding said gas puff into said vacuum chamber through a supersonic nozzle.  
     
     
       40. The method according to claim  36 , further comprising directing essentially the entire gas puff to the region adjacent said fast driving coil with essentially no transverse flow in the direction of the acceleration gap.  
     
     
       41. The method according to claim  36 , wherein said step of pre- ionizing further comprises inducing an electric field in the region of said gas puff for about  1  μs while maintaining electrical isolation between said fast driving coil and said gas.    
     
     
       42. The method according to claim  41 , wherein the step of pre- ionizing further comprising discharging an capacitor through said fast driving coil to generate an oscillating electric field.    
     
     
       43. The method according to claim  42 , further comprising generating an oscillating voltage in said fast driving coil having an oscillation period of less than about  1  μs.  
     
     
       44. The method according to claim  36 , further comprising the step of adjusting the position of the gas puff prior to the step of pre- ionizing.    
     
     
       45. The method according to claim  36 , wherein the step of moving said gas away from said fast coil comprises configuring the magnetic field in the region between the fast driving coil and the anode electrode to adjust the location of said magnetically confined plasma layer.  
     
     
       46. The method according to claim  45 , wherein the step of configuring the magnetic field comprises establishing a slow bias field using the fast driving coil.  
     
     
       47. The method according to claim  46 , wherein the step of creating a slow bias magnetic field further comprising discharging a bias field capacitor through said fast driving coil through a discharge circuit having a rise time of greater than about  1  μs.  
     
     
       48. The method according to claim  47 , wherein the step of fully ionizing comprises the step of driving an approximately  1  μs pulse to the fast driving coil.  
     
     
       49. The method according to claim  48 , wherein the step of driving further comprises discharging a fast capacitor through said fast driving coil to induce a voltage of about  20  kV on the pre- ionized gas to create said plasma.    
     
     
       50. The method according to claim  49 , further comprising the step of moving the plasma toward the anode electrode in about  1 . 5  μs or less.  
     
     
       51. The method according to claim  49 , further comprising the step of maintaining an insulating magnetic field in the acceleration gap prior to the step of applying said pulsed power signal.  
     
     
       52. The method according to claim  51 , further comprising the step of stagnating said plasma at a separatrix formed by the magnetic fluxes from said fast driving coil and slow driving coil prior to the step of applying said pulsed power signal.  
     
     
       53. The method according to claim  36 , further comprising the step of maintaining an insulating magnetic field in the acceleration gap prior to the step of applying said pulsed power signal.  
     
     
       54. The method according to claim  36 , wherein the step of extracting further comprises propagating the ion beam at least  20  cm through an essentially magnetic field free region.  
     
     
       55. The method according to claim  36 , further comprising the step of selecting the gas to be introduced from gas phase molecules selected from the group comprising hydrogen, helium, oxygen, nitrogen, fluorine, neon, chlorine, argon, lithium, beryllium, boron, carbon, sodium, magnesium, aluminum, silicon, phosphorous, sulfur, and potassium.